1. Technical Field of the Invention
The present invention relates in general to the mobile radio communications field and, in particular, to a method and system for use in determining the geographical position of a mobile radio terminal.
2. Description of Related Art
Mobile radio position determination is an emerging field that has gained a great deal of interest lately, and it is now desirable to include a position determination capability in future cellular mobile radio systems. The Time Difference of Arrival (TDOA) position determination method, which is known from military applications, has been used successfully for determining the position of mobile radio terminals. A typical TDOA position determination system can be either terminal based, whereby the Time Of Arrival (TOA) measurements are made on the "downlink" in the mobile station (MS), or network based, whereby the network performs the TOA measurements on the "uplink" in the radio base stations (BSs). These TOA measurements are then used to calculate TDOA parameters and estimate the MS's position.
One position determination system, which has been used for cellular mobile radio position determination, is marketed by TruePosition.TM.. This system has been used for determining the position of standard analog mobile radio terminals which operate in accordance with the IS-54 standard. Currently, these terminals constitute the vast majority of mobile radio terminals being used in the United States. The TruePosition system, with its own listening radio base stations, is operated independently of cellular systems and can serve wireline ("A" frequency band) and non-wireline ("B" frequency band) operators within the same geographical area. As such, these operators can share the same TruePosition position determination system. Upon request, the TruePosition system provides position information of individual cellular mobile radio terminals. Otherwise, the position determination system normally does not communicate with the cellular mobile radio systems.
As mentioned earlier, the TDOA method of position determination used by the TruePosition system is based on a known military application. Essentially, with the TDOA method, the absolute TOAs of an uplink message transmitted by a mobile radio terminal are registered in at least three fixed radio BSs. This information is processed in a centrally located processor, which calculates the position of the terminal. The registration of uplink messages in the TruePosition system is directed primarily to uplink control messages on the (analog) access channels (i.e., "reverse control channels" under the IS-54 standard). Notably, under the IS-54 standard, some of these control messages (e.g., registering messages and page response messages) contain the terminal identity in unencrypted code, which enables the TruePosition system to determine the position of a specific terminal without having to obtain any information from the cellular network operator responsible for the terminal concerned. The IS-54 standard further eases the positioning task by having all of the access channels assigned to a few, fairly narrow frequency bands, rather than having them dispersed over a wide frequency band among the traffic channels (e.g., as is the case for the IS-136 standard).
However, the primary use of access channels means that position determination is more easily performed for mobile radio terminals in an idle mode, because the access channels are used only by idle terminals (e.g. when registering or after being paged). If the position of a mobile terminal is to be determined while it is in a conversation mode, the TruePosition system has the option of utilizing a few traffic channels for voice channel tracking. Consequently, for example, if a police person's position is to be determined during an action while talking over a handheld radiophone, the network is required to hand-off or originally assign the radiophone to a traffic channel being monitored by the TruePosition system.
U.S. Pat. No. 5,327,144 to Stilp, et al. discloses a TDOA cellular telephone location system (apparently associated with the TruePosition system). According to this patent, the uplink signals transmitted periodically (e.g. cellular registering messages which can occur every 15 minutes under the IS-54 protocol) by a mobile radio terminal on a reverse (analog) control channel are received and recorded by at least 3 radio base stations. The TOA of each signal is recorded at the respective radio base station together with the identity of the transmitting terminal (contained in the uplink message). This information is transferred to a processor, which uses the TDOAs resulting from the three TOAs and the known locations of the radio base stations to calculate the position of the so-identified mobile radio terminal.
PCT Application No. WO 94/27161 to Stilp et al. (also apparently associated with the TruePosition system) discloses a TDOA system for determining the position of a mobile transmitter. The uplink signals transmitted responsively rather than periodically (e.g., cellular page-acknowledgment messages) by a mobile radio terminal are received, time-stamped with the TOA, and recorded by a plurality of BSs together with the identity of the transmitting terminal (contained in the uplink message). This information is transferred to a processor, which uses the TOAs and known locations of the radio BSs to calculate the position of the so-identified mobile radio terminal.
Although the above-described documents illustrate considerable progress in the cellular position determination field, there are still a number of problems left to be resolved. For example, the above-described TDOA method is generally not applicable to analog voice channels in an IS-54 system, because the voice channel frequencies are distributed to the radio BSs according to a frequency plan. If predetermined voice channels (frequencies) are to be used for position determination measurements instead of predetermined access channels, some radio BSs will be unable to assign voice channels on these predetermined frequencies because the BSs' combiners are tuned to base station-specific frequencies that may not include the predetermined frequencies. Consequently, the position of the terminals communicating with these radio BSs cannot be determined by the above-described method when those terminals are operating in the conversation mode.
An additional problem with the above-described positioning method in particular, when applied to a mobile radio terminal in the conversation mode on a traffic channel, relates to the power control of the mobile radio terminal. If the mobile terminal is near the serving radio base station, the land system will send power control orders to the mobile radio terminal requesting low uplink transmission power. More distant radio base stations ordered to measure TOA then will not receive a strong enough signal to perform the TOA measurements and read the terminal identity, if the distance from the mobile radio terminal is such that interference exceeds a certain limit.
Yet another problem with the above-described mobile terminal positioning method is that it is not applicable for digital voice channels in an IS-54 system, and the mobile terminal has to be handed over to one channel among a plurality of predetermined analog voice channels before the positioning method can be used. Furthermore, the above-described positioning method is difficult to apply under purely digital standards, such as in an IS-136 system (among others), because the control channels can be at any frequencies in the assigned frequency band, and it is impossible to record all traffic on all channels in advance before a position determination request has been received. Moreover, in some cellular mobile radio systems such as the digital Global System for Mobile communications (GSM), a frequency-hopping mode is used for traffic channels, which makes it virtually impossible to determine a mobile terminal's position according to the above-described method.
In conclusion, a need exists for a method of determining the position of any cellular mobile radio terminal on any uplink traffic channel or control channel and in accordance with any present and future mobile communication standards.